Wednesday, June 07, 2006

Number of days between two dates? (Java)

Recently a needed to get the number of days between two dates in Java.

Easy, right? Quite a few pages & articles suggest, and I admit my first iteration too, was:

Calendar firstDay = new GregorianCalendar(2006, Calendar.FEBRUARY, 3)
Calendar lastDay = new GregorianCalendar(2006, Calendar.JULY, 17);

static final long DAY_MS = 1000 * 60 * 60 * 24;
int days = (lastDay.getTime().getTime() - firstDay.getTime().getTime()) / DAY_MS;

It turns out this is WRONG, for example for the two dates given (days == 163, but shold be 164!) - some rounding error. This will round correctly, as some better Web pages explain:

double daysDouble = lastLong - firstLong;
int days = (int) Math.round(daysDouble / DAY_MS); // = 164

but using the Calendar API provides a clearer, more reable and most importantly correct version, too:

assert firstDay.get(Calendar.YEAR) == lastDay.get(Calendar.YEAR); // Assumption
int days = lastDay.get(Calendar.DAY_OF_YEAR) - firstDay.get(Calendar.DAY_OF_YEAR);

Or, for more calculations of this kind, consider http://joda-time.sourceforge.net/

5 Comments:

Anonymous Anonymous said...

I don't think this will work if the dates are in different years.

11 September, 2006 18:47  
Blogger Michael Vorburger said...

Absolutely, you are right; that's why I'd put that assert with // assumption in the code snippet above.

11 September, 2006 20:15  
Blogger tony said...

I have run into the same problem. My date ran is one hour less than expected. My date range like your spanned the start of daylight savings. I think that is the problem. You can't assume a constant MS/Day. The first day of daylight savings has only 23 hours.

Tony

10 March, 2008 16:38  
Anonymous Anonymous said...

What about this:

static final long ONE_HOUR = 60 * 60 * 1000L;

public long daysBetween(Date startDate, Date endDate) {
return ((endDate.getTime() - startDate.getTime() + ONE_HOUR) / (ONE_HOUR * 24));
}

or another one a bit different but same meaning:


private static final long MILISECONDS_PER_DAY = 24 * 60 * 60 * 1000;

public int getDaysBetweenDates(Date startDate, Date endDate) {
long diff = endDate.getTime() - startDate.getTime();
int days = (int) Math.floor(diff / MILISECONDS_PER_DAY);
return Math.abs(days);
}

David N. :)

20 May, 2008 15:34  
Anonymous Anonymous said...

here's a rather brutal way to do this :)

public static int getDays(Calendar from, Calendar to)
{
int cnt = 0;
while (from.before(to)) {
cnt++;
if (from.getActualMaximum(DATE) == from.get(DATE)) {
from.roll(MONTH, true);
}
if (from.getActualMaximum(MONTH) == from.get(MONTH)) {
from.roll(YEAR, true);
}
from.roll(DATE, true);
}
return ctn;
}

27 May, 2008 20:36  

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